This invention relates to a medical diagnostic ultrasound system and method for tissue motion analysis. In particular, techniques for improving estimates of motion from ultrasound data and characterizing the motion are provided.
Medical diagnostic ultrasound systems operate in various modes. Doppler tissue imaging mode is used to assist in diagnosis of heart tissue or other moving tissue structures. The ultrasound data is filtered with a clutter filter or other device to isolate information associated with tissue. The velocity of the tissue information is determined. The energy or variance of the tissue information may also be determined.
The velocities estimated from the ultrasound data represent velocities along one dimension. The velocities are estimated along an associated scan line, so may not represent the true magnitude of the velocity of the target tissue. To obtain more accurate representations of the actual velocity of the target, angle corrections are performed. In one system, the angle between the direction of movement of the target and the ultrasound beam is assumed to be towards a common point or location set by the user.
Other embodiments operate on velocities representing fluid flow. The user indicates a direction of flow of fluids, and the velocity of the fluids is determined as a function of the magnitude of velocity along each ultrasound line and the user input angle. For example, U.S. Pat. No. 5,785,655 discloses angle correction for velocity data along a line of study.
Tissue Doppler velocity information is used to compute the velocity gradient or strain rate. For example, a user inputs a series of connected line segments in a two-dimensional image. The ultrasound system calculates the strain rate as a function of velocities along those line segments.
The strain of tissue has been calculated from the strain rate. A time integral of the strain rate is typically started at an R-wave of a heart cycle and graphed through the cardiac cycle. Generally, only one component of the strain rate, such as a component along one direction, is integrated. Based on the assumption that all motion is in that one direction, the peak of the strain indicates the peak contraction of a piece of tissue. For healthy myocardial fibers at rest, the peak strain is typically around 16 percent, where the strain is multiplied by 100 and described as a percentage.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include methods and systems for accurate analysis of tissue motion based on ultrasound data. Motion of the ultrasound transducer is accounted for in estimates of tissue motion. Correcting for transducer motion better isolates localized tissue contractions or expansions, such as motion of the myocardial muscle or fibers. Accurate motion estimation is also provided by determining an angle of motion from the ultrasound data. The angle of motion is used to adjust velocity estimates, providing two-dimensional velocity vectors (i.e. motion estimates comprising motion in at least two dimensions). Movement of tissue is determined by correlating speckle or a feature represented by two different sets of ultrasound data obtained at different times. Decorrelation may be used to determine elevational motion.
Additional aspects include tracking the location of a tissue of interest. A characteristic of strain, such as the strain rate or strain, is calculated for the tracked tissue of interest. Ultrasound data associated with different positions relative to the transducer are selected as a function of the tracking and used to determine the characteristic of strain. Motion estimates corrected for transducer motion may also be used to determine a strain or strain rate.
In yet another aspect, motion estimates are generated with data from an intra-cardiac transducer array. The characteristic of strain is determined from the motion estimates. Other aspects discussed above may be used with an intra-cardiac transducer array, providing accurate motion analysis based on imaging from within the heart.
Further aspects and advantages of the invention are described below in conjunction with the preferred embodiments.